1. Field of the Invention
The invention relates to the peeling of logs into veneer for plywood production and more particularly to the application of supporting rollers to counteract the forces imparted by the peeling blade on a log during the peeling process.
2. Background Information
A log is peeled in a lathe which chucks the log ends and turns the log while a blade the length of the log is advanced against the log, e.g. at a rate of one-tenth of an inch for each revolution of the log, thereby peeling off one-tenth inch thick veneer. A nose bar is pressed against the log immediately prior to the blade. This bar is beneficial in that it compresses the wood fibers and assists the peeling process. Considering the pressure of both the blade and nose bar, a very significant force is applied to the log. This force becomes a problem as the log is peeled down to a diameter where it will bend or bow between the chucked ends in a direction away from the forces applied by the blade and nose bar. Such bending or bowing will cause the peeled veneer sheeting to thin out across the center and render the veneer defective.
The answer to this problem is to apply a counter force in the form of backup rollers on the side of the log opposite the direction of the applied forces of the blade and nose bar. A single roller has been determined to be inadequate in providing this counter force. The blade and nose bar forces are not directed in precisely the same direction. They are not equal forces nor is there a consistent relationship between the forces. (The blade will encounter soft and hard spots, e.g. knots in the log and thus the cutting force of the blade increases and decreases relative to the force of the nose bar.) The vector of the two forces thus sweep back and forth over an angular range. Thus, it is preferable that at least a pair of backup rollers are applied to the side of the log spanning this angular range.
In providing the backup rollers, certain objectives need to be considered. It is necessary that the rollers apply very high forces in order to counterbalance the forces of the blade and nose bar. It is also necessary that the rollers maintain their respective positions straddling the zone of the sweeping vector force. Finally, it is necessary that the rollers apply similar counterbalancing forces.
Two other factors come into play. The logs are loaded into the chuck of the lathe from the side opposite the blade and nose bar. This is, of course, where the backup rollers are to be positioned. The rollers are thus mounted on pivotal support arms that will swing the rollers out of the way to permit loading of the logs into the lathe chucks. Also, as the log is peeled, e.g. from 24 inches in diameter to a core of 4 inches in diameter, the rollers have to shift around the log periphery in order to maintain the balance of contact as between the rollers. Simply pivoting the pivotal arms to move the rollers continuously toward the decreasing log diameter does not achieve the required shifting of the rollers around the log periphery. One roller or the other, sometimes in alternating sequence, will be moved into a more direct contact with the log, while the other is moved away from the log.
Several methods have been adopted to accommodate the required shifting of the backup rollers to maintain both in contact with the log In one method, the pair of rollers are mounted on brackets that are independently pivoted relative to the support arms. As the support arms are pivoted, the brackets are also pivoted as necessary to achieve the desired shifting of the rollers. In a second method (depicted in U.S. Pat. No. 4,380,259), one of the rollers is mounted directly on support arms and the second roller is mounted on secondary support arms which is adjusted independent of the first roller.
In either of the above methods, the solution to the problem of adjusting the outboard backup rollers has required a menagerie of pivot arms, brackets and motors to achieve the desired backup pressure and thereby avoid log core bending. Because the forces are extremely high, the punishment placed on the mechanical moving parts is severe. A log will be peeled every few seconds and, thus, the many moving parts are in perpetual motion. The initial expense of this mechanism is very high and the repair costs are comparably high.